A preferred optical DRAW medium has a trilayer construction comprising a continuous reflecting layer on an optically-flat, disk-shaped, polymeric or glass substrate, a continuous, transparent optical spacer layer overlying the reflecting layer, and a continuous light-absorbing recording layer overlying the optical spacer. If the substrate itself is reflective, it may serve as the reflecting layer. A trilayer medium is schematically illustrated together with bilayer and monolayer media at page 25 of "Optical Disk Systems Image", by Bartolini et al., IEEE Spectrum, Vol. 15, No. 8, August 1978, pages 20-28. The layer or layers of those optical DRAW media and their thicknesses are chosen to minimize wasteful reflections and to maximize the absorption of the focused light beam at the light-absorbing layer. Thickness suggested for the light-absorbing layer are from about 5 to 50 nm.
U.S Pat. No. 4,097,895 (Spong) shows a bilayer medium comprising light-absorbing and reflecting layers. Its light-absorbing layer is a dye such as fluorescein which is vaporized by a focused light beam to form pits. U.S. Pat. No. 4,216,501 (Bell) shows a trilayer medium wherein the light-absorbing layer is a metal such as titanium or rhodium which is melted to form pits. U.S. Pat. No. 4,300,227 (Bell) shows a trilayer medium wherein the focused light beam causes an optical spacer layer to melt, sublime or decompose to produce gases which cause a bubble to form in a metallic light-absorbing layer. Thicknesses suggested for the light-absorbing layer are from about 2 to 25 nm. In the trilayer optical recording medium of U.S. Pat. No. 4,285,056 (Bell), information is recorded either as pits in both a metallic light-absorbing layer and an optical spacer layer, or as a bubble in the light-absorbing layer. The trilayer medium of European Patent Office application EP No. 58496 (Maffitt et al.) has a light-absorbing layer which is a refractory material, preferably amorphous carbon, boron, silicon, or alloys therebetween. Less preferred as the refractory material are borides, nitrides, carbides, oxides, and silicides. Useful are carbides of Al, Hf, Nb, Ta, Ti, V, W and Zr having a melting point in excess of 3000 K.
Although trilayer DRAW media discussed above are effective for use with argon laser and helium/neon laser recording systems, they are not as satisfactory for recording systems based on a laser diode which is the currently preferred light source, being much less expensive, having low power requirements, and providing very fast modulation speeds. Laser diodes typically emit recording wavelengths within the range of 700 to 900 nm.
Whatever the laser and whether or not the features are pits or bubbles, the ultimate objective is to create features such that when the recorded information is played back, the reproduced signals are essentially identical to the recorded signals, as evidenced by high signal-to-noise or carrier-to-noise ratios.